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Understanding the MK7 PCV function, retrofitting the MK8 PCV, and why catch cans are bullshit(IMO)

PerceivedShift

Autocross Champion
Location
Virginia
Install was pretty painless, here it is right after installing the hose, a little oil on the seals goes a long way.

2023-09-25 16.31.11.jpg

2023-09-25 16.02.09.jpg

2023-09-25 16.02.34.jpg


Looks fresh AF inside after 95k miles:
2023-09-25 15.59.53.jpg
 

MonkeyMD

Autocross Champion

MonkeyMD

Autocross Champion
Strengths: complaining

Opportunities : active listening
 

DerHase

Autocross Champion
Location
Hampton Roads, VA
Car(s)
2019 GTI Rabbit
Never got tracking info, but the TMD showed up today. I'm in the middle of track prep currently, but will attempt to install next week to start logging before I go on a cruise.

My prediction is it'll flow so much through the venturi that it requires tuning the WG (making it impractical for MOST people to want to do) if left unrestricted, but with an inline restrictor might be viable (but still require a retune). My concern is overspinning the turbo, but we'll see how much WG duty cycle I lose. If it's ~10% that'll be about equivalent of going from a catted DP back to stock for reference.
 

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PerceivedShift

Autocross Champion
Location
Virginia
So far I have a few normal driving miles on the MK8 PCV and this is what I have noticed so far. Though I haven't pushed a sustained 32psi on it yet.

-First thing I noticed right out the gate, which I wasn't expecting, is so far the car idles smoother...a lot smoother with both the A/C on and off. Was my old PCV so bad the windage at idle made all the difference?

-Usually on any short trip I'll see at least a blip of knock, with knock average being anywhere between -1.5 -> -0.75. Since the install knock average has been pegged at zero.
 

DerHase

Autocross Champion
Location
Hampton Roads, VA
Car(s)
2019 GTI Rabbit
So far I have a few normal driving miles on the MK8 PCV and this is what I have noticed so far. Though I haven't pushed a sustained 32psi on it yet.

-First thing I noticed right out the gate, which I wasn't expecting, is so far the car idles smoother...a lot smoother with both the A/C on and off. Was my old PCV so bad the windage at idle made all the difference?

-Usually on any short trip I'll see at least a blip of knock, with knock average being anywhere between -1.5 -> -0.75. Since the install knock average has been pegged at zero.

There's a number of ways the stock PCV can fail, and I don't think I've covered all of them in one place. Sounds like (in your old one) potentially the one way valve that prevents air from being sucked INTO the PCV at IDLE via the TIP could have been torn/leaking, essentially acting like a vacuum leak. Without seeing fuel trims etc that's just speculation - that's a kinda wild claim and likely placebo effect otherwise.

Or maybe there was a ton of oil inside the valve that was just slowly getting sucked in at idle (and if left to idle long enough, would have eventually idled "normal").
 

PerceivedShift

Autocross Champion
Location
Virginia
Before sometimes it would idle smoothly, but about 60% of the time it would idle rough enough to the point NVH bothered me at stop lights, especially with the AC on. Since I installed the MK8 PCV that 60% hasn't come back at any lights.

I'm tempted to do surgery on my old valve, for science?
 

MonkeyMD

Autocross Champion
So far I have a few normal driving miles on the MK8 PCV and this is what I have noticed so far. Though I haven't pushed a sustained 32psi on it yet.

-First thing I noticed right out the gate, which I wasn't expecting, is so far the car idles smoother...a lot smoother with both the A/C on and off. Was my old PCV so bad the windage at idle made all the difference?

-Usually on any short trip I'll see at least a blip of knock, with knock average being anywhere between -1.5 -> -0.75. Since the install knock average has been pegged at zero.
I noticed this too, but I did spark plugs at the same time, so figured it was that.
 

Mosquito

Go Kart Champion
Location
FL
As with anything else, there are a number of reasons why idle can be funky however you are not incorrect. There are a number of changes that generally go unnoticed because most often people don't perceive them as issues to look at in the first place or in some cases are accepted as VW things (cam cradle seal leak). Not everyone will have the same experience though, but there are a number of improvements that go well beyond the "smoke on track" issue.
 

DerHase

Autocross Champion
Location
Hampton Roads, VA
Car(s)
2019 GTI Rabbit
So since you guys have been in this from the beginning I'll share my preliminary findings from finally track testing it before I have a chance to get the info up on the website, as that'll take a few days to write everything coherently:

Zero issues with the car. No EPC lights etc like I had at VIR.

I was running on a much spicier map than I've run before - peaked at 26.5 to 27.5psi and tapers down to about 19psi at 6000rpm. I was seeing anywhere from 4.5 to 6 deg of timing at 6000rpms for reference, all on pump 93.

This is from my fastest (and most consistently fast) session - #2 which happened around 11:30 AM:



However when I was checking the logs, I noticed this which alarmed me (though there was no reported smoke or drivability problems).

Near the end of session #3:

1696211076760.png


Full session for reference, the above is zoomed in on the single big blip towards the right in this full session:

1696211123384.png


HOWEVER - when I overlaid the data on video it became clear it did not happen under typical circumstances. The knock occurs simultaneously while hitting a curb HARD and tipping in on the throttle. I'm 98% positive this is "false knock" and can be ignored. There is logic in the ECU that will do exactly this under severe circumstances.

The little blips here and there are all no more than -2.25 on any single cylinder and entirely tune related. I probably won't make any real changes to it, as my knock sensors are barely numbed and I'll leave them to do their job as they're supposed to do.

 
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